Dynamic B₀ field shimming for improving pseudo‐continuous arterial spin labeling at 7 T

Abstract

PurposeB0 field inhomogeneity within the brain‐feeding arteries is a major issue for pseudo‐continuous arterial spin labeling (PCASL) at 7 T because it reduces the labeling efficiency and leads to a loss of perfusion signal. This study aimed to develop a vessel‐specific dynamic B0 field shimming method for 7 T PCASL to improve the labeling efficiency by correcting off‐resonance within the arteries in the labeling region.MethodsWe implemented a PCASL sequence with dynamic B0 shimming at 7 T that compensates for B0 field offsets in the brain‐feeding arteries by updating linear shimming terms and adding a phase increment to the PCASL RF pulses. Rapidly acquired vessel‐specific B0 field maps were used to calculate dynamic B0 shimming parameters. We evaluated both 2D and 3D variants of our method, comparing their performance against the established global frequency offset and optimal encoding scheme‐based corrections. Cerebral blood flow (CBF) maps were quantified before and after corrections, and CBF values from different methods were compared across the whole brain, white matter, and gray matter regions.ResultsAll off‐resonance correction methods significantly recovered perfusion signals across the brain. The proposed vessel‐specific dynamic B0 shimming method improved the labeling efficiency while maintaining optimal static shimming in the imaging region. Perfusion‐weighted images demonstrated the superiority of the 3D dynamic B0 shimming method compared to global or 2D‐based correction approaches. CBF analysis revealed that 3D dynamic B0 shimming significantly increased CBF values relative to the other methods.ConclusionOur proposed dynamic B0 shimming method offers a significant advancement in PCASL robustness and effectiveness, enabling full utilization of 7 T ASL high sensitivity and spatial resolution.